What an idiot!!!
I just cracked it. The answer was staring me in the face all along!
All I had to do was to raise the upper figure in the HorizSync line in /etc/X11/XFreeConfig-4 to 81.0 so the driver could use the 80.3kHz rate in order to sync with the 75.3Hz Vertical Refresh.
Thanks, though, Andy, at least your answer got me thinking along the right lines...